Method for solution mining of uranium ores

ABSTRACT

In the solution mining of uranium ores using an aqueous ammonium carbonate leaching solution containing hydrogen peroxide and/or molecular oxygen as oxidant, permeability of the ore formation during the leaching operation is maintained or improved by including a small amount of alkali metal silicate dissolved in the leaching solution. The silicate also improves the stability of the oxidant in many instances.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of copendingapplication Ser. No. 739,967, filed Nov. 8, 1976, now abandoned.

DESCRIPTION TECHNICAL FIELD

The present invention relates to processes for the solution mining ofuranium ores and more particularly to such processes which maintain thepermeability of the uranium ore formations.

BACKGROUND ART

With the increasing use of nuclear power plants for the production ofelectricity in the United States, uranium ore deposits have become anincreasingly valuable natural resource. Even though there are extensiveuranium deposits distributed throughout the Western United States, manyof these are located at too great a depth from the surface and/or are oftoo low concentration to be mined economically by conventional open pitor shaft mining techniques. Especially for such ore sources whereconventional mining techniques are uneconomical or where they presentsevere ecological or esthetic problems, solution mining has beenproposed in many instances.

In a typical solution mining situation, a central production well can bedrilled into a permeable uranium ore formation and a plurality ofregularly spaced injection wells drilled around the production well. Tostart production, a leaching solution is pumped into the ore formationthrough the injection wells. The solution moves through the formationdissolving the uranium compounds in the ore as it passes toward thecenter of the ore formation from which it is removed by means of theproduction well. The leaching solution containing the dissolved uraniumis then pumped to an extraction treatment zone where the leachingsolution is treated to separate the uranium compounds.

Several solution mining (in-situ leaching) processes have beensuggested. For example, the solvent most frequently used for leachinghas been an acid or carbonate solution. The uranium is then removed fromthe leaching solution by ways such as (1) adjusting the pH of thesolution to neutral or basic pH to precipitate out the uranium, (2)separating the uranium compounds by ion exchange or (3) concentratingthe uranium by liquid-liquid extraction.

Many in-situ leaching operations employ an alkaline carbonate leachingsolution containing an oxidizing agent. The carbonate can be present asan ammonium or sodium salt or mixtures thereof. Ammonium ions arepreferred in many instances because they are less likely to interferewith permeability of the ore formation.

Because uranium in the 4+ valence state is insoluble in water, anoxidant is needed to oxidize it to the 6+ valence state, which issoluble in the form of a carbonate complex. The basic chemistry of thismethod of extraction is shown by the following equations:

    UO.sub.2 +1/2O.sub.2 →UO.sub.3                      ( 1)

    UO.sub.3 +H.sub.2 O+3CO.sub.3.sup.-- →UO.sub.2 (CO.sub.3).sub.3.sup.4- +2OH.sup.-                        ( 2)

The hydroxyl ions produced in reaction (2) tend to cause formation ofinsoluble uranium compounds, especially when sodium ions are alsopresent. The -OH ions can, however, be readily removed by reaction withbicarbonate ions which favorably affect the equilibrium of thesolubilizing reaction as well as prevent precipitation of insolubleuranium compounds such as sodium uranate. Thus, it is usually preferredto use carbonate leaching solutions containing enough bicarbonate toreact with hydroxyl ions formed in the manner of reaction (2).

Though many oxidizing agents have been suggested and tried for this use,hydrogen peroxide and molecular oxygen (O₂) are especially desirable forthis use because they and their decomposition products--O₂₂ and H₂O--are completely non-polluting and thus ecologically acceptable.Hydrogen peroxide is preferred, however, because it can be introduced asa liquid that contains oxidant in highly concentrated form, whereas theconcentration of injected oxygen gas is highly limited by itssolubility. As a consequence, the liquid oxidant is less likely than agas to cause vapor locking within the ore body. Even when the hydrogenperoxide does decompose in contact with the ore, the O₂ produced islikely to be well distributed over a wider portion of the ore body inthe form of quite small sized bubbles which further contribute to aneven more thorough distribution of oxygen solubilized in leach solution.Thus, there is greater potential for increasing the reaction rates forsolubilizing the insoluble uranium compounds in the ore.

The chemistry of uranium leaching is less well characterized forhydrogen peroxide than for oxygen. Conceivably, by analogy with equation(1) above, the reaction may be:

    UO.sub.2 +H.sub.2 O.sub.2 →UO.sub.2.sup.++ +2OH.sup.-

However, uranium in the 6+ valence state is known to form peroxyaddition compounds such as UO₆ ⁻⁻, and it is entirely likely that one ormore peroxy compounds are involved in the overall chemistry. Suffice itto say, however, that H₂ O₂ is a potential oxidant either as H₂ O₂ or asa latent source of O₂.

The use of either molecular oxygen or hydrogen peroxide in alkalinecarbonate leaches may contribute to the tendency of the formation tobecome less permeable as the leaching process proceeds. Diminishedpermeability greatly increases the time required to leach out an orebody. Thus, the use of hydrogen peroxide and/or molecular oxygen as theoxidant, along with other factors such as the particular cations in theleaching solution, the type of clay, the electrostatic charges on theclay particles and pressure drop between the injection and productionwells, in some instances appear to cause loss of permeability.

The problem of maintaining permeability of an ore body during leachingis not a new one. For example, in U.S. Pat. No. 3,309,140, Gardner et alpropose the addition of polyacrylamide to an acidic leaching solutioncontaining sodium chlorate as an oxidant. U.S. Pat. No. 3,567,427mentions that hydrogen peroxide can be effective for the disaggregationof certain clay minerals, which suggests that hydrogen peroxide might betroublesome in applications such as solution mining where disaggregationis to be avoided. These references obviously do not, however, addressthemselves to the problem of maintaining permeability in the presence ofhydrogen peroxide. De Vries in U.S. Pat. No. 3,908,388 discloses the useof the reaction product of a non-aqueous slurry of alkali metal silicatewith an alkyl amide to insolubilize the alkali metal silicate for thepurpose of stabilizing sand and thus to maintain oil permeability. Also,Peeler in U.S. Pat. No. 2,968,572 employs similar amides to insolubilizeaqueous alkali metal silicates for soil stabilization in the presence ofground moisture. However, the problem contemplated there was oilpermeability, not water permeability. Furthermore, no oxidant waspresent in the system and higher silicate concentrations were used.

Several other U.S. patents disclose the use of many other agents forgelling or setting alkali metal silicates to make them useful for soilstabilization, e.g., the following: .

U.S. Pat. No. 583,166 Portland cement

U.S. Pat. No. 3,288,040 Alkali metal hexafluorosilicate

U.S. Pat. No. 3,558,506 Methyl C₁₋₃ acylates.

The use of alkali metal silicates for the purpose of soil stabilizationhas heretofore apparently been limited to systems in which the alkalimetal silicate was admixed with an extraneous agent for the purpose ofgelling or solidifying the dissolved silicate.

DISCLOSURE OF THE INVENTION

According to the present invention there is provided in a process forthe solution mining of a uranium ore formation where an aqueouscarbonate leaching solution containing an oxidant is passed through theore formation to dissolve uranium from the formation therein and thesolution is withdrawn from the ore formation enriched in uranium, theimprovement comprising: passing through the ore formation an aqueousammonium carbonate leaching solution having a pH of 7-10, and which hasdissolved therein an oxidant selected from the group consisting ofhydrogen peroxide, molecular oxygen and a mixture thereof and an alkalimetal silicate.

The process of the invention is applicable generally to the use ofammonium carbonate leaching solutions which contain either hydrogenperoxide or molecular oxygen as an oxidant. When hydrogen peroxide isused, the process is likely to involve both species of oxidant since thehydrogen peroxide undergoes decomposition in contact with the ore. Suchdecomposition is probably catalytic in nature, can be quite extensiveand may be virtually complete in some instances.

As used herein, the term "ammonium carbonate leaching solutions" meansaqueous solutions of NH₃ and CO₂. Such solution will ordinarily have aninitial pH, i.e., prior to injection into the ore formations, of 7-10.Although the concentration of NH₂ and CO₂ may vary widely within thosepH limits, the leaching solution will ordinarily be comprised of from0.5 to 20 grams per liter of ammonium carbonates, basis either --CO₃ or-HCO₃. From 1 to 10 grams per liter are preferred. Such solutions areprepared most easily by sparging NH₃ and CO₂ into water until thedesired concentrations of chemical reactants are reached and thenadjusting the pH by the addition of more NH₃ to raise the pH or CO₂ tolower pH. They can, however, also be made by dissolving the solidcarbonates in the water and then adjusting pH in the same manner onlyaltering the proportions of the solid carbonates. Typically, an ammoniumcarbonate leaching solution will contain from about 10 to 15 grams ofcarbonate compounds per liter of solution, e.g., 10 grams of ammoniumbicarbonate and 0-5 grams of ammonium carbonate.

The concentration of hydrogen peroxide in the leaching solution is onlyone factor that impinges on successful solution mining of uraniumbearing ores. Other economic or technical parameters associated with theparticular formation being treated, such as pH, particle size andtemperature, may be more important and may even be overridingconsiderations. Ordinarily, however, the aqueous leaching solution willcontain 0.1-10 grams H₂ O₂ per liter and preferably 0.2-2 grams H₂ O₂per liter.

Hydrogen peroxide suitable for use in leaching solutions used in theinvention is available commercially in aqueous solutions containing from10 to 90% by weight H₂ O₂, any of which can be used in the invention. Asused in accordance with the invention, hydrogen peroxide has very littleeffect on pH of the leaching solution and therefore need not ordinarilybe a factor in adjusting pH of the leaching solution. These H₂ O₂solutions may in some instances contain one or more stabilizers toinhibit decomposition such as those which are disclosed in U.S. Pat.Nos. 2,872,293; 3,122,417; 3,387,939; 3,649,194; 3,691,022; 3,687,627and 3,869,401. However, the use of such stabilizers is not essential tothe practice of the invention.

It has been found that only a very small concentration of alkali metalsilicate per liter of total leaching solution is needed to reduce lossof permeability within the formation significantly. The minimumeffective concentration of silicate is highly subjective to theformation being treated and its particular physical and chemicalcharacteristics. However, ordinarily at least about 0.1 and preferablyat least about 0.2 gram of alkali metal silicate will be used per literof total leaching solution. However, more significant effects areproduced if at least about 0.5 gram per liter is used. An optimumconcentration of alkali metal silicate appears to be 0.5-1.5 grams perliter. Even through higher concentrations of alkali metal silicate(e.g., up to 5 g/l) may be used, no further advantage with respect topermeability was apparent from such use. Furthermore, the use of higherconcentrations in some instances will significantly increase theincidence of gelling the silicate which will cause a loss inpermeability of the ore formation. However, it has been found thatleaching solutions containing as high as 10 grams of NH₃ -CO₂ per literand even higher and the preferred 0.5-1.5 grams of silicate per literresist gellation for quite long periods of time.

Suitable alkali metal silicates include silicates of sodium, potassiumand lithium, of which sodium is preferred. The silicates must, however,be stable aqueous solutions which contain no appreciable amount ofparticulate silica. Transparent solutions which exhibit little, if any,Tyndall effect, are uniformly suitable with respect to stability againstloss of permeability induced by gellation. Suitable aqueous sodiumsilicate solutions are available having SiO₂ :Na₂ O weight ratios offrom 1.90 to 3.25 and containing from 27.0 to 36.0% wt SiO₂ and from 8.7to 19.4% wt Na₂ O. Sodium silicate solutions of this type are alkalineand have a pH range of between 10 and 13.

In preparing leaching solutions for use in the process of the invention,no particular order of mixing is needed.

The invention is exemplified and can readily be understood by referenceto the examples which are set out hereinbelow.

DEFINITIONS AND ABBREVIATIONS

ABC=ammonium bicarbonate=NH₄ HCO₃

AC=ammonium carbonate=(NH₄)₂ CO₃

NH₃ -CO₂ or "carbonate" refers broadly to aqueous leach solutions of NH₃and CO₂ containing ammonium carbonate, ammonium bicarbonate or mixturesthereof.

"Goal flow" or "goal flow rate" refer to a predetermined flow rate to bemaintained during a run (within the capabilities of the pump being used)to pump leachate from the bottom of a leach column.

"Companion Run" refers to side-by-side comparative column leaching runs.

"Leach" refers to the solution fed from the inlet reservoir to the topof the leach column.

"Leachate" refers to the solution pumped from the bottom of a leachcolumn after passing through the ore bed.

The terms "silicate", "sodium silicate", "sil.", and "NaSiO₃ " may beused interchangeably. All weights or concentrations are on the basis ofDu Pont Sodium silicate, Grade F or Grade No. 9 diluted to the samesolids as Grade F. Both Grades have an SiO:Na₂ O weight ratio of 3.25.

EXPERIMENTAL APPARATUS AND PROCEDURE

1. Apparatus

Two parallel leaching systems were set up, each having (1) an inletreservoir for fresh leaching solution, (2) a leach solution feed pump onthe outlet of the inlet reservoir communicating with (3) a leachingcolumn containing a fixed bed of finely divided uranium ore having adepth in most cases of about 2.5-10 cm, (4) a peristaltic leachate pumpon the outlet of the leach column discharging into (5) a leachatereservoir. The vapor space in the tops of the leach columns, theleachate reservoir and inlet reservoir were each manifolded to a gascollection burette so that any O₂ gas release in the system could bemeasured.

2. Procedure

(1) Pack leach columns with ore charge resting atop a glass wool plug ona coarse fritted glass disk. Tamping was generally not required toprevent voids;

(2) Charge inlet reservoirs with one liter of leaching solution withingredients being added in the following order: NH₄ HCO₃ and/or (NH₄)₂CO₃, sodium silicate solution and H₂ O₂ solution;

(3) Pump leaching solution into leach columns to permeate to bottom ofore bed and continue pumping rate;

(4) Activate leachate pump and establish as nearly as possible apredetermined leach flow through both systems. Pump speeds wererecorded.

3. Ore Characteristics

A. A high-uranium ore containing 0.85% wt U and about 21% wt CaCO₃ froma South Texas site. Material was dry (1-1.4% wt H₂ O based on dryingloss at 110° C.) and free flowing.

B. A weakly mineralized ore of sandy consistency containing only about0.03% wt U rich in pyrite also from South Texas. Material wassufficiently wet (12-14% wt H₂ O) that it did not flow freely but waseasily spooned into the leach column.

C. A very weakly mineralized ore containing less than 5 ppm by weight U.Material was dry and free flowing.

4. Methods of Determining H₂ O₂ Loss During Leaching

A. By Gas Collection

As described above, manifolded flexible tubing conveyed all O₂ gasreleased by H₂ O₂ decomposition from the leach column, the inletreservoir and the leachate collection reservoir into an inverted gascollection cylinder. The percent of H₂ O₂ decomposed (% converted to O₂)after a time (t) in which a certain volume (liters) of leachate,containing a certain concentration of H₂ O₂ was collected, was computedas follows: ##EQU1## The collected gas volume was converted to STP bymultiplying the observed volume by 0.9. This conversion factor was basedon the finding that when 1.8 g of H₂ O₂ was contacted with threedifferent ores and the released gas collected by displacement of water,the following relation was obtained: ##EQU2## B. By Titration

Assuming that the H₂ O₂ in the inlet reservoir is stable, the H₂ O₂ lostsolely to leaching can be measured by titrating a grab sample from thecolumn. This was generally done by collecting at the end of a run(without interrupting flow) a 100 cc sample of leachate at the same flowrate (mostly 5 cc/min) as used during the run. Then 20 cc aliquots ofthe 100 cc sample and of the leach in the inlet reservoir were titratedby standard iodimetry: ##EQU3## Unless otherwise stated, analysis of "H₂O₂ lost by titration" is by titration of a grab-sample rather than ofthe collected leachate.

EXAMPLE I

This example illustrates both the effect of a leaching solutioncontaining H₂ O₂ in reducing permeability of an ore body and thereversal of that effect by adding aqueous sodium silicate to theleaching solution.

Using Ore A, a control leach solution containing 10 g ABC/l, 2.5 g AC/land 1.2 g H₂ O₂ /l (pH 8.7) was pumped through a 50 g ore bed containedin a 42 mm ID column, at 10 cc/min. After 20-30 minutes, the columnbegan to plug, and by 50-60 minutes it was only possible to pump fromthe bottom of the ore bed about 5 cc/min, even though the outfall pumprate was substantially increased. Stirring the wet bed with a spatuladid not improve permeability.

In a companion set of two runs, it was found that silicate essentiallyprevented this loss of permeability. With 3 g/l of silicate (Du PontGrade F) in the abovedescribed leach solution, a flow rate of 10 cc/minwas easily maintained over a 65-minute period, to collect 650 cc ofleachate. Using no silicate in the companion run, it took 98 minutes tocollect 620 cc of leach, even though the outlet pump was at a muchhigher speed.

EXAMPLE II

In a series of tests, permeability of the ore bed was examined as afunction of H₂ O₂ in the leaching solution, pH and silicate in theleaching solution. One hundred grams of Ore C were extracted incompanion 42 mm leaching columns using a basic leach solution containing10 g ABC/l +2.5 g AC/l adjusted to pH 8.6 with NH₃ or to pH 10.2 withNaOH. Goal flow rate was 5 cc/min. The results are given in Table 1.

                  TABLE 1                                                         ______________________________________                                        Cumulative                                                                            Basic Leach                                                           Running Modifications                                                         Times,             Column   Leach Flow Rate, cc/min                           Min.    Column A.sup.(1)                                                                         B.sup.(2)                                                                              Column A Column B                                 ______________________________________                                         0-60   as is      as is    5 cc/min at                                                                            Similar                                          (no H.sub.2 O.sub.2)                                                                     (no      25 rpm pump                                                          H.sub.2 O.sub.2)                                                                       speed                                             60-68   as is      as is    12.8 cc/min                                                                            Similar                                          (no H.sub.2 O.sub.2)                                                                     (no      at 58 rpm                                                            H.sub.2 O.sub.2)                                            68-108 1.8g H.sub.2 O.sub.2 /1                                                                  1.8g     Slowed to                                                                              Similar                                                     H.sub.2 O.sub.2 /1                                                                     4-5 cc/min                                                                    at >110 rpm                                                          also                                                       108-144 1.8g H.sub.2 O.sub.2 /1                                                                  added    3.6 cc/min                                                                             6.1 cc/min                                                  3g       at 50 rpm                                                                              at 50 rpm                                                   NaSiO.sub.3 /                                                                 1                                                                             (Example                                                                      III)                                                       ______________________________________                                         .sup.(1) Leaching solution pH 8.6                                             .sup.(2) Leaching solution pH 10.2                                       

Summary of Results:

With no H₂ O₂ in either formulation, the pump setting needed to pull 5cc/min from the bottom of the columns was close to the 20-25 rpm used tomaintain an inlet feed of 5 cc/min to the columns, indicating only asmall resistance to flow. With a modest increase in pump speed, the ratequickly rose to 12.8 cc/min.

At both pH 10.2 and 8.6 the addition of H₂ O₂ during the runs caused avery noticeable loss in the rate at which leach solution could be pulledthrough the columns. A further indication of permeability loss was shownby the reading of a vacuum gauge at the outlet of the column beingleached at pH 8.6, which showed only 0.6 in. during the period when noH₂ O₂ was in the leach. However, the vacuum gradually increased to 20-21in. of mercury during the 40-minute leach period after H₂ O₂ was added.

Note in the last 36-minute time period that the addition of silicate tothe pH 10.2 leach solution caused a marked improvement in leach flow ascompared to the pH 8.6 leach containing no silicate.

In another similar run (using one column), the vacuum at the outlet ofthe column rose to 16.8 in. over a 40-minute period. As the NH₃ -CO₂leach containing 1.8 g H₂ O₂ /l was pumped from the bottom of the columnat an average rate of 4.0 cc/min, the pump speed had to be increasedfrom 23 to 130 rpm (85 rpm after 7 minutes). Then 3 g NaSiO₃ /l wasadded to the leach and pumping was resumed for 60 minutes to pump 298 ccof leach (5.0 cc/min). During this time the vacuum decreased from 18.5to about 13.5 in. as the pump speed also gradually decreased to about 70rpm. The leach flow was interrupted for ten minutes, and during anensuing 15-minute flow period, an average 6.3 cc/min flow rate wasobtained as the pump speed fell to 42 rpm and the vacuum fell to about 7in.

EXAMPLE III

In this test series, the adverse effect of H₂ O₂ on permeability and itsprevention by use of sodium silicate addition were demonstrated on Ore Busing a basic leach containing 4 g ABC/l+4 g AC/l. The results are givenin Table 2.

                                      TABLE 2                                     __________________________________________________________________________                      Basic Leach   Leach Flow Rate                                                                           Pump Speed                                 Length of Run                                                                          Modifications cc/min      rpm                               Run No.  Min.     Column A                                                                             Column B                                                                             Column A                                                                            Column B                                                                            Column A                                                                            Column                      __________________________________________________________________________                                                      B                           1 (Control)                                                                            Run, 60 min                                                                            None   --     2.83  --    58    --                                            (no H.sub.2 O.sub.2)                                        2 (Control)                                                                            Run, 90 min                                                                            None   --     2.89  --    58 → 64                                                                      --                                            (no H.sub.2 O.sub.2)                                                 Stand overnight.                                                     3 (Control)                                                                            Run, 60 min                                                                            Add 1.80g                                                                            --     1.67        64 → 119                                     H.sub.2 O.sub.2 /1                                                   Recharge columns                                                              with fresh ore.                                                      4A (Control)                                                                           Run, 110 min                                                                           with 1.80g                                                                           --     1.27  --    115 after                                                                           --                                            H.sub.2 O.sub.2 /1 & no   20 min                                              sil                                                         4B (Example III)                                                                       Run, 110 min                                                                           --     with 1.80g                                                                           --    2.76  --    58 → 110                                      H.sub.2 O.sub.2 /1 plus  over 110                                             1g sil/1                 min                                  Stand for 49 min;                                                             no flow.                                                             5A (Control)                                                                           Run, 40 min                                                                            with 1.80g                                                                           --     0.87  --    115   --                                            H.sub.2 O.sub.2 /1 & no                                                       sil                                                         5B (Example III)                                                                       Run, 40 min                                                                            --     With 1.80g                                                                           --    2.77  --    100                                                  H.sub.2 O.sub.2 /1 plus                                                       1g sil/1                                             __________________________________________________________________________

EXAMPLE IV

In a series of tests, the effect of the following variables uponpermeability was studied: (1) adding H₂ O₂ with and without silicate;(2) discontinuing silicate addition; and (3) effect of adding silicateafter permeability has been diminished. Two comparison runs were run ata goal flow rate of 10 cc/min on Ore A using a basic leach containing 10g ABC/l+2.5 AC/l.

In two comparison runs, Columns A and B were first flushed with 500 ccof peroxide-free basic leach for 50 min. A flow of 10 cc/min was easilymaintained well below a pump speed of 100 rpm. When 1.20 g H₂ O₂ /l wasadded to the leach solution to Column A, and pumping from the column wasresumed for 30 minutes, the outlet pump speed had to be increased to 345rpm during this period to be able to maintain an overall flow rate ofclose to 10 cc/min. However, this same flow rate could be obtained inthe companion column (B), in which the peroxide-containing leach alsocontained 3 g/l of sodium silicate, at a pump speed of only 92 rpm. Whenthe silicate was removed from the leach being fed to Column B, facileflow (at 10 cc/min) was maintainable for an additional 2 hours, at theend of which the run was terminated. These runs show not only thatsilicate is beneficial in preventing loss of permeability, but also thatthe beneficial effect is sustained for a substantial period of timeafter silicate is removed from the leaching solution.

In a similar companion set of runs, partial plugging was induced bypumping through 320 cc of a silicate-free basic leach solutioncontaining 1.20 g H₂ O₂ /l for 56 minutes, as a result of which flowdiminished to below 5 cc/min even though pump speed was increased to 350rpm. When 3 g/l of sodium silicate was added to the leach, flow rate didnot improve as the next 210 cc of leach was pumped through the ore.However, when pumping was interrupted for 72 minutes, it became possibleto pump 1000 cc of leach through the ore bed over an 80 minute period atabout 12.5 cc/min at a pump speed of only about 85 rpm. In the companioncontrol run, where silicate was not added, the 72-minute standing periodcaused only a temporary relief from plugging. Over the subsequent80-minute period, flow was less than half the 12.5 cc/min rate of thesilicate run even though pump speed was up to about 425 rpm.

The second experiment indicates that permeability, even after beingpartially lost during leaching with a peroxide-containing ammoniumcarbonate solution, can be restored if silicate is added to the leachsubsequent to the loss. Furthermore, this experiment suggests that suchrestoration of permeability is best effected by interrupting the flow ofleach for a period of time after the ore has been permeated with arelatively small quantity of silicate-containing leach.

EXAMPLE V

In a series of tests, the following effects were examined:

(1) Loss of permeability with peroxide leach

(2) Use of silicate to prevent loss of permeability, and

(3) Use of silicate to restore permeability once lost.

Table 3 is a running log of comapanion leach experiments using the basicleach 4 g ABC/l+4 g AC/l (equivalent, by calculation to 2.28 g NH₃ /land 4.06 g CO₂ /l) +1.80 g H₂ O₂ /l, and 100 gram charges (in the 42 mmI.D. columns) of ore A. Information on permeability is given by the"Leachate Flow Rate" column in Table 3 in conjunction with data on pumprpm's below in the text. Information on H₂ O₂ lost during leaching isbased on O₂ loss (last column in table).

                                      TABLE 3                                     __________________________________________________________________________    COLUMN LEACHING OF ORE A                                                      EFFECT OF SILICATE AND STABILIZER ON STABILITY AND PERMEABILITY               BASIC LEACH: 4 g ABC/1 + 4 g AC/1 + 1.80 g H.sub.2 O.sub.2 /1                                                         LEACH-                                                                  VOL   ATE                                             LENGTH                  LEACH-                                                                              FLOW                                            OF                      ATE   RATE                                            RUN   pH     ADDITIVES  cc    cc/min                                RUN NO.   MIN   A  B   A    B     A  B  A  B                                  __________________________________________________________________________    6A & B (Control)                                                                        90    8.6                                                                              8.6 None None  455                                                                              450                                                                              5.1                                                                              5.0                                7A & B (Control)                                                                        60    8.6                                                                              8.6 None None  135                                                                              100                                                                              2.2                                                                              1.7                                8A & B (Control)                                                                        37    8.6                                                                              8.6 None None   25                                                                               22                                                                              0.67                                                                             0.59                                         Stand overnight, no flow                                            9A (Control)                                                                  9B (Example)                                                                            50    8.6                                                                              8.77.sup.(1)                                                                      None 4 g/l Sil.sup.(2)                                                                    52                                                                              160                                                                              1.0                                                                              3.2                                          Stand overnight for 62 min, no flow.                                10A (Control)                                                                 10B (Example)                                                                           35    8.6                                                                              8.77                                                                              None 4 g/l Sil                                                                            35                                                                              208                                                                              1.2                                                                              5.9                                          Let stand 156 min., then add silicate to A and run A alone for                next 74 min.                                                        11A (Example)                                                                           74    8.77                                                                             --  4 g/l Sil                                                                          --    111                                                                              -- .75.sup.(3)                                                                      --                                           Let stand 272 min., then run B alone for next 30 min.               12B (Example)                                                                           30    -- 8.77                                                                              --   4 g/l Sil                                                                           -- 162                                                                              -- 5.4                                          Let stand over weekend, then run A alone for next 41 min.           13A (Example)                                                                           41    8.77                                                                             --  4 g/l Sil                                                                          --    336                                                                              -- 8.2                                                                              --                                 14A & B (Example)                                                                       36.5  8.77                                                                             8.77                                                                              4 g/l Sil                                                                          4 g/l Sil                                                                           150                                                                              150                                                                              4.1                                                                              4.1                                15A & B (Example)                                                                       100   8.77                                                                             8.77                                                                              4 g/l Sil                                                                          4 g/l Sil                                                                           415                                                                              418                                                                              4.2                                                                              4.2                                __________________________________________________________________________     .sup.(1) pH after silicate and H.sub.2 O.sub.2 added; note small pH           raising effect of silicate.                                                   .sup.(2) Sodium silicate with SiO.sub.2 *Na.sub.2 O = 3.25. Source Du Pon     Grade F.                                                                      .sup.(3) Excluding the first 3 minutes, where flow rate was 19.3 cc/min. 

The following can be concluded from these data

Permeability gradually diminished over the 187 minutes (Control Runs6-8) for the ammonium carbonate/bicarbonate leach containing H₂ O₂ andno silicate, even though the pump speed had increased to <110 rpm.However, permeability was largely restored when 4 g/l of silicate wasadded to the leach (Example 9B). In this regard, compare the 35-minuteand 50-minute running periods of Runs 10 and 9, but note that a no-flowor rest interval (62 min) was needed after the silicate-bearing leachwas added during the 50-minute period. The 5.9 cc/min rate (Example Run10B) was obtained at a pump speed of 62 rpm; the comparative 1.22 cc/minrate (Control Run 10A) was obtained at 115 rpm.

When silicate was then added to the silicate-free column that had beenused in Control Run 9A, there was a marked improvement in permeability.Note, however, (Example Run 11A) that permeability was not improvedimmediately. Other experiments indicated that 30-60 minutes (andpossibly less) was a sufficient interlude between first introducing thesilicate-bearing leach and resuming flow.

EXAMPLE VI

The following tests were carried out to determine the effectiveness oflow levels of sodium silicate addition to the leaching solution.

In two companion runs using 100 gram charges of Ore C, one ore columnwas leached with a solution containing 4 g ABC/l+4 g AC/l+1.8 g H₂ O₂/l, the other with the same solution fortified with 0.2 g/l of sodiumsilicate. Leachate was pumped from both column outlets using the samepump rpm. The leach containing 0.2 g/l of silicate flowed noticeablybetter, about 1.3 times faster than the leach containing no silicate forthe first 80 minutes, and increasing to 1.44 times faster during thenext 120 minutes.

When the leachate containing no silicate was then enriched with 0.5 g/lsodium silicate, the flow improved to the point that it was as good as,or slightly better than, the leach containing 0.2 g/l of sodiumsilicate.

In another 60-minute comparative run, leach containing 1 g/l of sodiumsilicate flowed easily at 5 cc/min at pump settings of 45-65 rpm. Withno silicate the average flow over this period was only 3.6 cc/min, eventhough the pump speed was increased from 65 to 118 rpm during the run.

EXAMPLE VII

Because of interest in using sodium as well as ammonium carbonates,tests were conducted in which the ammonium ion was replaced in part orcompletely by sodium.

Table 4 summarizes companion runs using different leaches containing1.80 g H₂ O₂ /l and 100 gram charges of Ore C in the 42 mm I.D. column.A goal leach flow-rate of 5 cc/min using a pump having 0-120 rpm rangewas attempted.

The ore had been sieved to -12+80 mesh, but the particles were soft (tofinger crushing) and clay-like in appearance. When wetted in the column,the larger pieces seemed to lose their particulate identity and thewetted plug seemed to be a fairly uniform, sandy, clay-like aggregate.

                                      TABLE 4                                     __________________________________________________________________________    SILICATE AND PERMEABILITY IN COLUMN LEACHING: EFFECT OF Na.sup.+  VS          NH.sub.4.sup.+  IN LEACH                                                                              ADDITIONS        LENGTH                               RUN.sup.(1)     g/l NaSiO.sub.3                                                                       TO ADJUST pH                                                                            pH     OF RUN,                                                                             LEACH FLOW RATES.sup.(6)       NO.   BASIC LEACH                                                                             A   B   A    B    A   B  MIN   A       B                      __________________________________________________________________________    16    4 g NH.sub.4 HCO.sub.3 /l +                                                             0   1.0 None None 8.6 8.65                                                                             60    3.6 cc/min                                                                            OK, at 45-65                 4 g (NH.sub.4).sub.2 CO.sub.3 /l         at 65 → 118                                                                    rpm                                                                   rpm                            17    4 g NH.sub.4 HCO.sub.3 /l +                                                             1.0 1.0 H.sub.2 SO.sub.4                                                                   NaOH 7.0 9.6                                                                              70    OK at 38-66                                                                           3.6 cc/min at                4 g (NH.sub.4).sub.2 CO.sub.3 /l         rpm     80-110 rpm             18.sup. (2)                                                                         4 g NH.sub.4 HCO.sub.3 /l +                                                             1.0 1.0 5 g/l                                                                              None 8.6 8.6                                                                              140   OK at 22-43                                                                           Flow gradually               4 g (NH.sub.4).sub.2 CO.sub.3 /l                                                                Na.sub.2 SO.sub.4      rpm     slowed to 3.5                                                                 cc/min at 110                                                                 rpm                    19.sup.(3)                                                                          4 g NH.sub.4 HCO.sub.3 /l +                                                             1.0 --  None --   8.6 -- 57    OK at ˜ 32                                                                       --                          4 g (NH.sub.4).sub.2 CO.sub.3 /l         rpm                            20    4 g NH.sub.4 HCO.sub.3 /l +                                                             1.0 1.0 0.60 g/l                                                                           4.3 g/l                                                                            9.0 9.6                                                                              110   OK at 35-118                                                                          OK at 35-68                  4 g (NH.sub.4).sub.2 CO.sub.3 /l                                                                NH.sub.3                                                                           NH.sub.3          rpm.sup.(7)                                                                           rpm                    21    4 g NH.sub.4 HCO.sub.3 /l +                                                             1.0 1.0 1.1 g/l                                                                            3.0 g/l                                                                            9.0 9.6                                                                              40    OK at 25-80                                                                           OK at 25-49                  4 g (NH.sub.4).sub.2 CO.sub.3 /l                                                                NaOH NaOH              rpm     rpm                    22.sup.(4)                                                                          8 g (NH.sub.4).sub.2 CO.sub.3 /l                                                        0   2.0 None None 8.78                                                                              8.80                                                                             A:130 Fell off                                                                              OK at ˜ 25                                                B:60  2.6 cc/min                                                                            rpm                                                                   during last                                                                   60 min at                                                                     117 rpm                        23.sup.(2),(5)                                                                      8 g (NH.sub.4).sub.2 CO.sub.3 /l                                                        0   2.0 None None 8.78                                                                              8.80                                                                             110   Fell to 2.2                                                                           OK at ˜ 30                                                      cc/min at                                                                             rpm                                                                   111 rpm                                                                       during last                                                                   80 min                         24    8 g (NH.sub.4).sub.2 CO.sub.3 /l                                                        2.0 2.0 None None 8.78                                                                              8.80                                                                             140   OK at ˜ 25                                                                      OK at ˜ 25                                                      rpm during                                                                            rpm                                                                   last 100                                                                      min                            25    8 g (NH.sub.4).sub.2 CO.sub.3 /l                                                        1.0 1.0 4.7 g/l                                                                            13.8 g/l                                                                           10.0                                                                              10.0                                                                             40    OK at 55                                                                              1.2 cc/min at                                  NaOH NH.sub.3          rpm     111 rpm                26    4 g NH.sub.4 HCO.sub.3 /l +                                                             1.0 1.0 3.9 g/l                                                                            3.85 g/l                                                                           9.6 9.6                                                                              70    OK at ˜ 25                                                                      1.3 cc/min at                4 g (NH.sub.4).sub.2 CO.sub.3 /l                                                                NH.sub.3                                                                           NaOH              rpm     110 rpm                27    4 g NH.sub.4 HCO.sub.3 /l +                                                             1.0 1.0 3.9 g/l                                                                            3.85 g/l                                                                           9.6 9.6                                                                              60    OK at ˜ 25                                                                      1.2 cc/min at                4 g (NH.sub.4).sub.2 CO.sub.3 /l                                                                NH.sub.3                                                                           NaOH              rpm     110 rpm                28    4 g NH.sub.4 HCO.sub.3 /l +                                                             1.0 1.0 13.8 g/l                                                                           5.05 g/l                                                                           10.0                                                                              10.0                                                                             90    OK at ˜ 25                                                                      1.5 cc/min at                4 g (NH.sub.4).sub.2 CO.sub.3 /l                                                                NH.sub.3                                                                           NaOH              rpm     114 rpm                29    8 g (NH.sub.4).sub.2 CO.sub.3 /l                                                        1.0 1.0 9.3 g/l                                                                            4.7 g/l                                                                            10.0                                                                              10.0                                                                             110   OK at ˜                                                                         OK at                                          NH.sub.3                                                                           NaOH              25-35 rpm                                                                             25-35 rpm              30    10 g NaHCO.sub.3 /l +                                                                   0   2.0 None None 8.78                                                                              8.95                                                                             60    1.3 cc/min                                                                            1.5 cc/min                                                            at ˜ 110                                                                        at 95 →                                                                113                                                                   and then                                                                              rpm                                                                   < 0.1 cc/min                   31    5 g NaHCO.sub.3 /l +                                                                    0   1.0 None None 8.75                                                                              9.02                                                                             130   OK at 20-45                                                                           3.0 cc/min at                0.25 g Na.sub.2 CO.sub.3 /l              rpm     ˜ 111 rpm;                                                              then down to                                                                  0.3 cc/min             32    10 g NH.sub.4 HCO.sub.3 /l +                                                            3.0 3.0 NH.sub.3                                                                           NaOH 8.6 10.2                                                                             70    OK at 23-77                                                                           1.4 cc/min at                2.5 g (NH.sub.4).sub.2 CO.sub.3 /l       rpm; mostly                                                                           115 rpm after                                                         at 54   15                     __________________________________________________________________________                                                           min                     .sup.(1) Brackets denote runs using same ore charge.                          .sup.(2) After standing 2-3 days after previous runs.                         .sup.(3) After standing 90 min, after previous run.                           .sup.(4) Prior to this, leached with 600 cc of H.sub.2 O.sub.2 --free         leach at 5 cc/min.                                                            .sup.(5) Prior to this, leached with 965 cc of H.sub.2 O.sub.2 --free         leach at 5 cc/min.                                                            .sup.(6) "OK" means a 5 cc/min flowrate is maintainable at the pump speed     (rpm's) indicated.                                                            .sup.(7) Flow improved greatly (only 30-56 rpm needed) for last 40 minute     of this run, which followed a 44min noflow interlude.                    

The data in Table 4 show the following.

(1) Without silicate, permeability decreased during column leaching.Silicate in an NH₃ -CO₂ -H₂ O₂ leach improved permeability (Runs 16, 22and 23).

(2) Silicate improved permeability throughout the pH range 7-10 normallyassociated with in-situ uranium leaching. Some effect was seen even whentotal NH₃ was >10 g/l (e.g., Run 28).

(3) In Runs 30 and 31 using a sodium carbonate/bicarbonate leach,silicate did not improve permeability. (Note, however, that inherentpermeability using the sodium-based leach was also less than that ofammonium-based leach cited in the table.)

(4) In most runs where a substantial concentration of NaOH (e.g.,3.85-5.05 g/l) was added to the leach to raise a pH to 9.6 or 10.0, thebeneficial effect of silicate on permeability was lost (See Runs 17 and25-28). In fact, these caustic-fortified leaches seemed to cause evenpoorer permeability than silicate-free leaches containing no caustic.(Compare these last cited runs with Runs 16, 22 and 23.)

In a minority of runs (Runs 21 and 29), however, the causticfortification did not seem to interfere with permeability. Furthermore,note, in Run 18, that the addition of 5 g/l of Na₂ SO₄ caused noapparent loss in permeability. The conclusions to be drawn from theseobservations are that, for purposes of improving permeability in acarbonate leach containing H₂ O₂,

(a) silicate is decidedly beneficial in an NH₃ -CO₂ system in a pH rangeat least as broad as pH 7-10,

(b) that silicate may be beneficial in a carbonate leach containing bothNH₄ ⁺ and Na⁺ as counterions for carbonate and bicarbonate, and,

(c) that carbonate/bicarbonate leaches containing only Na⁺ counterionsappear to cause more permeability loss than silicate-containing NH₃ -CO₂formulations whether or not they contain silicate.

EXAMPLE VIII

In this series of tests, a comparison was made between the use of H₂ O₂as oxidant, both with and without silicate, and NaClO₃ as oxidantwithout silicate.

Test Conditions:

Two companion runs at goal flow 5 cc/min.

Basic Leach: 4 g ABC/l+4 g AC/l; pH 8.6

Column A: 3 g NaSiO₃ /l+1.8 g H₂ O₂ /l

Column B: 10.8 g NaClO₃ /l

Ore: 100 g/column, containing about 0.85% uranium

                                      TABLE 5                                     __________________________________________________________________________    ORE PERMEABILITY: HYDROGEN PEROXIDE VS SODIUM CHLORATE                                                VOL.                             Uranium                  RUN.      OXIDANT & LEACHATE                                                                             LEACH FLOW        % LOSS                                                                                Extracted.sub.2      RUN TIME                                                                              pH    ADDITIVE  cc     RATE, cc PUMP SPEED                                                                             BY O.sub.2                                                                            Mg.sup.(1)           NO. MIN.                                                                              A  B  A    B    A  B   A   B    rpm      A   B   A  B                 __________________________________________________________________________    33  80  8.72                                                                             8.6                                                                              H.sub.2 O.sub.2 +                                                                  NaClO.sub.3                                                                        400                                                                              400 5.0 5.0  23-112                                                                            23-34                                                                              113 --  221                                                                              187                             3 g/l Sil                                                       34  70  8.72                                                                             8.6                                                                              H.sub.2 O.sub.2 +                                                                  NaClO.sub.3                                                                        398                                                                              398 5.0 5.0  ˜ 54                                                                        ˜ 32                                                                         98  --   79                                                                              41                              3 g/l Sil                                                       35  120 8.72                                                                             8.6                                                                              H.sub.2 O.sub.2 +                                                                  NaClO.sub.3                                                                        600                                                                              600 5.0 5.0  ˜ 50                                                                        ˜ 50                                                                         96  --  127                                                                              69                              3 g/l Sil                                                       25      Flush with 150 cc of basic leach and let stand overnight, no                  flow.                                                                 36  170 8.6                                                                              8.6                                                                              H.sub.2 O.sub.2                                                                    NaClO.sub.3                                                                        850                                                                              850 5.0 5.0  49-85                                                                             25-30                                                                              92  --  100                                                                              41                              only                                                                    Let stand 75 min, no flow.                                            37  140       H.sub.2 O.sub.2                                                                    NaClO.sub.3                                                                        700                                                                              700 5.0 5.0  38-101                                                                            22-24                                                                              93  --   95                                                                              31                38  50        only      192                                                                              195 3.8 3.8  115 ˜ 15                                                                         --  --  -- --                7.5     Flush with basic leach (68 cc).                                               Stand over weekend, no flow.                                          39  60        H.sub.2 O.sub.2 +                                                                  H.sub.2 O.sub.2                                                                    250                                                                              238  4.17                                                                             4.0  20-73                                                                             23-> 85                                                                            67  67  -- --                              0.5 g/l                                                                            only                                                                     Sil                                                             40  60        H.sub.2 O.sub.2 +                                                                  H.sub.2 O.sub.2 +                                                                  210                                                                              134 3.5 2.2  20-112                                                                            21-114                                                                             --  --  -- --                              0.5 g/l                                                                            1 g/l                                                                    Sil  Sil                                                                Let stand 70 min, no flow.                                            41  150       H.sub.2 O.sub.2 +                                                                  H.sub.2 O.sub.2 +                                                                  740                                                                              745 4.9 5.0  25-40                                                                             40-75                                                                              52  63  -- --                              0.5 g/l                                                                            1 g/l                                                                    Sil  Sil                                                        __________________________________________________________________________     .sup.(1) Determined by dibenzoyl methane colorimetry.                    

Summary of Results:

The following conclusions can be drawn from the data in Table 5.

Loss of permeability is not associated with all oxidants. Note theexcellent permeability with sodium chlorate, Runs 33-37B. On the otherhand, the preceding examples showed that H₂ O₂ -containing leach incontact with ore caused loss of permeability. Furthermore, note from thegas collection data in Runs 33-37A that virtually all the H₂ O₂decomposed to O₂ gas during passage over the ore bed. Thus, it wouldappear that the loss of permeability is occasioned by H₂ O₂ and/or theattendant oxygen resulting from decomposition of the H₂ O₂ in contactwith the ore.

Runs 33-35 clearly show that a silicate fortified peroxide leach is alsofreely flowing. Both the chlorate and peroxide-silicate leaches flowedfreely for 270 minutes, requiring on the average a pump speed of wellbelow 50 rpm's. It is interesting to note here that the beneficialeffects from silicate were sustained for a substantial period of timeafter silicate was removed from the leaching solution. However, thebeneficial effects gradually fell off after silicate was removed fromthe leaching solution. See Runs 36A-38A following Runs 33A-35A. Compareespecially the 50-minute running period in Runs 38A, B.

Though (non-plugging) chlorate is also a potential oxidant for solutionleaching of uranium ore, it does suffer from the disadvantage ofproducing far more environmentally objectionable reaction products, suchas chlorite and chloride ions. Furthermore, it is a less effectiveoxidant for leaching uranium as shown by the data in the last column.This was so even though twice as many moles of chlorate than H₂ O₂ wereused, i.e., 0.101 vs. 0.053 moles/liter of each.

EXAMPLE IX

It was noted that when a leachate containing 10 g ABC/l+2.5 g AC/l+3 gsil/l was allowed to stand one or more days, a slight but quitenoticeable haze or light cloudiness developed which indicates gelling ofthe silicate. This was even more noticeable by the Tyndall effect from abeam of light. After fortifying with 1.2 g H₂ O₂ /l, this aged solutionwas pumped through 50-100 g ore beds, which resulted in a marked loss ofpermeability for Ores B and C after less than 3 hours of leaching. Thereis no such loss when freshly made solution free of gellation is used.

EXAMPLE X

Following up on the findings of Example IX, the gellation stability of anumber of leach compositions having varying salt contents was observedwith respect to variations in silicate content. Hellige Turbidimeterdata for the several leaches are given in Table 6 below:

                  TABLE 6                                                         ______________________________________                                        Gellation Stability                                                                                 A.P.H.A. TURBIDITY                                      COM-                  UNITS.sup.(2) (ppm)                                     POSI- LEACH           AFTER                                                   TION  COMPOSITION, g/l                                                                              6       11                                              NO..sup.(1)                                                                         ABC    AC     SIL  pH.sup.(1)                                                                         DAYS   DAYS                                     ______________________________________                                        1     --     16     1.0  8.95 1.3   --                                        1A    --     16     1.0  8.40 --    2.2                                       2     --     8      1.0  8.90 nil   0.2                                       2A    --     8      1.0  8.20 --    0.3                                       3     --     4      1.0  8.90 0.1   nil                                       3A    --     4      1.0  8.20 --    nil                                       4     8      8      1.0  8.80 1.3   1.2                                       4A    8      8      1.0  8.20 --     0.95                                     5     4      4      1.0  8.80  0.70 nil                                       5A    4      4      1.0  6.60 --    0.1                                       6     4      4      0.5  8.80 0.3   nil       CLEAR                           6A    4      4      0.5  7.00 --    nil                                       7     4      4      2.0  8.85  0.95 0.6                                       7A    4      4      2.0  7.20 --    1.2                                       8.sup.(3)                                                                           8      8      1.0.sup.(3)                                                                        8.80 1.2   0.9                                       8A.sup.(3)                                                                          8      8      1.0.sup.(3)                                                                        7.20 --    1.2                                       9.sup. (3)                                                                          4      4      1.0.sup.(3)                                                                        8.80 1.2   1.4                                       9A.sup.(3)                                                                          4      4      1.0.sup.(3)                                                                        7.30 --    nil                                       10.sup.(3)                                                                          4      4      2.0.sup.(3)                                                                        8.80 1.7   1.4                                       10A.sup.(3)                                                                         4      4      2.0.sup.(3)                                                                        6.80 --    0.4                                       11    10     2.5    3.0  8.55 very  gel on                                                                  cloudy                                                                              bottom                                                                        of beaker                                 12    10     2.5    3.0  8.55 very  gel on                                                                  cloudy                                                                              bottom                                                                        of beaker                                 ______________________________________                                         .sup.(1) For A series, each numbered sample was split at 6 days and the A     series portion was adjusted downward with H.sub.2 SO.sub.4. Thus, only th     last 5 days of the 11day readings (last column) were at the lowered pH fo     A series solutions.                                                           .sup.(2) These values are from calibration curve supplied by Hellige for      SiO.sub.2, and do not necessarily represent the actual ppm of SiO.sub.2 i     these leaches.                                                                .sup.(3) From an aged (many months old) lab supply of sodium silicate tha     had sediment on the bottom. All other runs used silicate from a new drum      of Du Pont Grade F sodium silicate.                                      

During this work it was observed that cloudiness (and thus, gelling) ofleach solutions could be completely avoided by using 2.0 g/l or less ofsilicate in a solution not too concentrated in salts, e.g., 10 g/l.Within the range pH 7-8.95, it didn't seem to matter much where the pHwas. At pH 10 and above, silicate is inherently free from gellingbecause of solubilization by base.

Overall, for the clear solutions, the only suggestion of possible orincipient gelling, as manifested by somewhat higher turbidity readings,was for compositions containing high leach salt content (CompositionsNumber 1, 1A, 4, 4A, 8 and 8A), high silicate (Compositions 7, 7A and10) or aged silicate (Compositions 8-10).

Fortunately, the salt and silicate concentrations giving apparentlynon-gelling leaches coincide with the concentrations which are highlyuseful for leaching and prevention of permeability loss.

EXAMPLE XI

In this Example, extended runs were carried out which illustrate thatsilicate may in some instances be effective to stabilize the H₂ O₂solution from decomposition.

In a companion run using 100 gram charges of Ore C, a 5 cc/min flowrate, and a basic leach containing 10 g ABC/l+2.5 g AC/l+1.80 g H₂ O₂ /l(pH 8.2), the comparison tested the difference in adding or not adding 3g NaSiO₃ /l to the leach. The run was for 330 minutes, and permeabilityloss was prevented by prior screening so the ore contained -12 +80 meshparticles. The results can be summed up as follows:

    ______________________________________                                                       Loss of O.sub.2 in cc/min                                      Serial Time      Without  With                                                Segments         Silicate Silicate                                            ______________________________________                                        First 30 min     --       --                                                  2nd 100 min      2.4      1.05                                                3rd 100 min      2.65     1.15                                                4th 100 min      3.05     1.05                                                TOTAL - 330 min  2.80     1.26                                                Stand overnight  --       --                                                  with no flow                                                                  100              2.35     0.95                                                ______________________________________                                    

The O₂ loss for the 330 minutes corresponded to an 84% loss of H₂ O₂ inthe leach when the leach contained no silicate; only 37% when the leachcontained silicate.

These results showed that silicate not only improved H₂ O₂ stabilityduring leaching, but also seemed to prevent an increase in instabilitywith time.

We claim:
 1. In a process for the solution mining of a uranium oreformation where an aqueous carbonate leaching solution containing anoxidant is passed through the ore formation to dissolve uranium from theformation therein and the solution is withdrawn from the ore formationenriched in uranium, the improvement comprising preventing permeabilityloss by passing through the ore formation, which otherwise would therebyhave a permeability loss, aqueous ammonium carbonate leaching solutionhaving a pH of 7-10, and which has dissolved therein an oxidant selectedfrom the group consisting of hydrogen peroxide, molecular oxygen and amixture thereof and a small amount of alkali metal silicate effective torestore the permeability.
 2. The process of claim 1 wherein the leachingsolution contains 0.1-5 grams per liter of alkali metal silicate.
 3. Theprocess of claim 2 wherein the leaching solution contains 0.1-10 gramsper liter of H₂ O₂ and 0.5-20 grams per liter of ammonium carbonates. 4.The process of claim 3 wherein the alkali metal silicate is sodiumsilicate present in the solution at a concentration of 0.5-1.5 grams perliter.
 5. The process of claim 3 wherein the silicate-containingleaching solution is passed through the ore formation after anonsilicate-contaning leaching solution has been passed through the oreformation resulting in reduced permeability of the ore formation.